地球用英文介绍

The Earth's surface is very young.In the relatively short (by astronomical standards) period of 500,000,000 years or so erosion and tectonic processes destroy and recreate most of the Earth's surface and thereby eliminate almost all traces of earlier geologic surface history (such as impact craters).Thus the very early history of the Earth has mostly been erased.The Earth is 4.5 to 4.6 billion years old,but the oldest known rocks are about 4 billion years old and rocks older than 3 billion years are rare.The oldest fossils of living organisms are less than 3.9 billion years old.There is no record of the critical period when life was first getting started 71 Percent of the Earth's surface is covered with water.Earth is the only planet on which water can exist in liquid form on the surface (though there may be liquid ethane or methane on Titan's surface and liquid water beneath the surface of Europa).Liquid water is,of course,essential for life as we know it.The heat capacity of the oceans is also very important in keeping the Earth's temperature relatively stable.Liquid water is also responsible for most of the erosion and weathering of the Earth's continents,a process unique in the solar system today (though it may have occurred on Mars in the past).